GB1590247A - Method for producing lithographic printing plates - Google Patents

Description

PATENT SPECIFICATION (l

( 21) Application No 5007/78 ( 22) Filed 8 Feb 1978 ( 31) Convention Application No 767017 ( 32) Filed 9 Feb 1977 in ( 33) United States of America (US) ( 44) Complete Specification Published 28 May 1981 ( 51) INT CL 3 GO 3 C 1/94 1) 1 590 247 ( 52) Index at Acceptance G 2 C 1 E 1 B CZA ( 54) A METHOD FOR PRODUCING LITHOGRAPHIC PRINTING PLATES ( 71) We, POLYCHROME CORPORATION, a corporation organized and existing under the laws of the State of New York, United States of America, of 137 Alexander Street, Yonkers, State of New York, 10702, United States of America.

(Assignee of EUGENE GOLDA, LEONARD WILKES and SIMOND LONG CHU) do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:

This invention relates to a method for producing aluminium lithographic printing plates.

In the production of aluminium presensitized lithographic printing plates, it has been found beneficial to treat the surface of the aluminium substrate sheet, with a protective interlayer substance which imparts beneficial characteristics to the final lithographic printing plate thus produced.

Many prior art disclosures relating to the production of lithographic plates do not show the inclusion of an interlayer, but plates manufactured according to these disclosures demonstrate an inadequate adhesion of the photosensitive material to the base substrate and are unsatisfactory for long press runs The prior art teaches that it is desirable to treat the aluminium sheet substrate surface receiving the light sensitive coating material, which when exposed to light and developed becomes the printing surface of the printing plate, with an interlayer substance which forms a strong bond with the aluminium sheet substrate and with the light sensitive coating material.

Many such interlayer substances are known in the art for manufacturing longerrunning lithographic plates U S Patents No 3,160,506; No 3,136,636; No.2,946,683; No 2,922,715 and No.

2,714,066 disclose a variety of suitable interlayer materials and methods for applying them Alkali metal silicate, silicic acid, alkali metal zirconium fluoride and hydrofluozirconic acid solutions presently are the most important commercial interlayers substances Interlayers formed from these substances substantially improve the bonding of the light-sensitive coating to the underlying aluminium base which otherwise generally tends to have inadequate affinity for the coating Of the various known interlayer substances, the Group IV-B metal fluorides are preferred In particular, the alkali metal zirconium fluorides, such as potassium zirconium hexafluoride, and hydrofluozirconic acid disclosed in U S Patents No 3,160,506 and No 2,946,683 are especially satisfactory for preparing anodized aluminium bases to receive a light-sensitive coating.

Heretofore the interlayer substance has usually been applied to the aluminium sheet substrate by immersing the sheet in a solution of the interlayer substance This requires that an immersion tank be employed, and depending upon the rate of speed of the moving web of the metal substrate, the length of detention time within the immersion tank can be varied.

However, because an immersion tank of substantial size is required to obtained a satisfactory treatment with the interlayer substance such processes have a relatively high energy requirement.

It has been proposed in U S Patent 2,922,715 issued to Gumbinner, and in U S.

Patent 2,946,683 issued to Mellan and Gumbinner to use an organic acid such as citric acid or tartaric acid to effect hardening of the interlayer surface.

It has also been proposed in U S Patent 2,714,066 issued to Jewett and Case to effect a simple drying of a coating of silicate solution disposed on a foil.

It has now been found that if a solution of orl us 1 590 247 the interlayer substance is applied solely to and directly on the surface of the aluminium sheet substrate which is to be employed for use as a lithographic printing plate, in sufficient amount to provide for complete reaction with the surface to give a satisfactory bonding coating to the aluminium sheet substrate and the thus applied solution is then subjected to a heating treatment at elevated temperature whereby to increase the concentration of the interlayer substance on the surface of the aluminium sheet substrate and bring the reaction between the solution and aluminium to completion, a coated substrate is formed with markedly improved properties for the production of lithographic printing plates Thus the bonding and protecting ability of the interlayer thus produced is improved resulting in an increase in the life of lithographic printing plates produced therefrom and, by the avoidance of an immersion tank a significant decrease in the capital equipment required is achieved.

Furthermore, in addition to providing an interlayer of improved properties, the contact time of from 15 seconds to several minutes depending on the interlayer used which was formerly required to prepare a lithographic substrate to accept a light sensitive coating is significantly reduced.

A number of undercoating or interlayer treatments are known in the art.

The present invention provides a method of producing a lithographic printing plate which comprises applying an aqueous coating solution of an alkali metal silicate, silicic acid, alkali metal zirconium fluoride, hydrofluozirconic acid, Group IV-B metal fluoride, or polyacrylic acid, to at least one surface of an aluminium sheet, removing therefrom any excess solution thereafter subjecting the coated aluminium sheet to a hot air baking treatment at an elevated temperature and for a time sufficient to complete the reaction of the coating solution which has not reacted with the surface of the aluminium sheet and applying a light sensitive lithographic composition to the coated aluminium sheet.

As a result, there is a considerable reduction in the amount of production time, energy and capital equipment required for producing a lithographic printing plate having an interlayer.

As the first step in the process of this invention, a sheet substrate of aluminium especially those aluminum compositions suitable for the manufacture of lithographic printing plates such as Alcoa 3003 and Alcoa 1100 which may or may not have been pre-treated by standard graining and/or etching and/or anodizing techniques as are well known in the art, is coated by spraying, brushing, dipping or other means with an aqueous coating solution of one of the aforesaid interlayer substances Standard aluminium substrate pretreatments include electrolytically anodizing in sulfuric, chromic, hydrochloric and/or phosphoric acids, electrolytically etching in hydrochloric or phosphoric acid, and chemical or mechanical graining by well known methods, which are all known to the skilled worker Coating solutions empolyable in the practice of this invention include aqueous solutions of alkali silicate, silicic acid, the Group IV-B metal fluorides, polyacrylic acids, the alkali metal zirconium fluorides such as potassium zirconium hexafluoride, or hydrofluozirconic acid in concentrations of 0 5 % to 20 % by volume A preferred concentration range is from 3 % to 8 % and the most preferred range is from 4 % to 5 %.

Excess solution is then removed from the surface of the metal substrate, for example by doctoring, and the resulting coated plate is then subjected to baking at an elevated temperature, for example, in an oven, typically from 50 degrees C to 300 degrees C for from 5 to 120 seconds, whereby a completely reacted interlayer is formed.

A preferred elevated temperature range is from 80 degrees C to 200 degrees C and the most preferred elevated temperature range is from 100 degrees C to 150 degrees C It is preferred to subject the coated substrate to the elevated temperatures for a period of from 10 to 45 seconds and most preferred from 15 to 20 seconds Subsequently, the surface may be treated for example, by rinsing with water to remove any excess unreacted materials The resulting coated aluminium sheet substrate may then be treated with a photosensitive composition suitable for use as a lithographic printing plate, such as a positive or negative acting diazo composition, for example, paradiazo di-phenolamine condensed with formaldehyde and optionally mixed with ink receptive polymers suitable to produce a presensitized lithographic printing plate, as is well known in the art.

The superior results obtained from the practice of the instant invention is evidenced by a comparison of the treated interlayer of the instant invention with a substrate coating obtained by using standard techniques disclosed in the prior art To this end a standard zincate test as described in U S.

Patent 3,940,321 at column 3 line 36, et seq.

is performed on each type surface The zincate test is a measure of the protection which the interlayer affords the base substrate from extraneous eroding compounds and is a measure of the completeness of the reaction between the coating solution and the substrate Such comparative testing consistently shows a marked superiority in protection and stability of the substrate of 1 590 247 the instant invention obtained by a subsequent baking of an applied interlayer at elevated temperatures as compared to substrates prepared by standard techniques used in the art.

Compositions which are generally employed as interlayers in lithographic plates actually are not completely pure materials.

A variety of impurities are present in the substances employed for such purposes, which either prevent or delay the consummation of the reaction between the intended interlayer material and the aluminium sheet substrate This causes an instability and unpredictability in the finished product since the intended reaction does not go to completion under controlled conditions For example, a lithographically suitable aluminium plate which has been anodized with an aluminum oxide sub-stratum may be further coated with a protective silicate bonding layer by a method represented by the equation, Al + 3 A 102 + 2 Si O 2 2 (A 125 i O 5) However, there are other compounds present in the interlayer film such as, aluminum hydroxide, Al(OH)3 and hydrated sodium aluminum silicates such as, Na 2 O A 1203 2 Si O 2 6 H 2 which delay or prevent the completion of the above reaction under normal conditions Periodic zincate tests conducted on a sample lithographic plate prepared in accordance with this invention over an extended interval of time produced consistent zincate readings whereas a similar test on a substrate coated by prior art techniques, produced increasing readings indicating a gradual rather than instantaneous completion of the reaction over time The method of this invention drives the above reaction to completion immediately and forces the removal of the impurities by the intended interlayer reactant, thus yielding a final product of improved storage stability and quality.

As a result, it has been found that on the average, lithographic plates made in accordance with the present invention display a 33-1/3 % to 50 % increase in shelf life and a % to 33 1/3 % increase in press life, with a marked improvement in the tenacity between the interlayer and the photosensitive coating.

The following examples are provided to illustrate the operation of the present invention and in no way limits its scope, Example 1

Two sets of mechanically grained aluminum sheets were anodized by use of direct current in a sulfuric acid solution by a method well known in the art The plates were then treated as follows:

A series of aluminum plates "A" to be treated by the method of this invention were spray coated with a 4 % aqueous sodium silicate solution at room temperature, excess was doctored off and the plates then subjected to a hot air baking treatment at degrees C for 15 seconds The plates were then rinsed to remove excess reactants and a light sensitive lithographic coating applied to the surface of each treated plate.

A series of aluminium plates "B" were dipped in a 5 % sodium silicate solution which was maintained at 180 degrees F for 2 minutes The plates were then rinsed and a light sensitive lithographic coating applied to the surface of each treated plate.

Each plate was exposed to accelerated shelf life testing which showed plates made according to treatment A of the instant invention to have consistent good image producing quality after a simulated 18 month shelf life whereas plates made according to treatment B demonstrated inconsistent image producing quality after a 12 month simulated shelf life This test demonstrated that the "A" plates shelf life was superior to that of the "B" plates by at least 50 %.

Plates A and B were imaged and developed according to well known methods and mounted on a printing press Plate A showed first appearance of image wear after 180,000 impressions, whereas plate B showed first appearances of image wear after 150,000 impressions, indicating the improved characteristics of the "A" plates.

Example 2

Treatments and tests were run similar to Example 1 except the sodium silicate was replaced by a 1 % aqueous solution of potassium zirconium fluoride Similar results were obtained.

Example 3

Treatments and tests were run similar to Example 1 except the aluminum sheets were not anodized Similar results were obtained.

Example 4

Treatments and test were run similar to Example 1 except the aluminum plates were chemically etched in a 5 % solution of trisodium phosphate instead of mechanical graining Similar results were obtained.

Example 5

Treatments and test were run similar to Example 1 except the sodium silicate con 125 centration was 6 5 % and the baking treatment of plate A was for 22 seconds at 100 degrees C Similar results were obtained.

4 1 590 2474 Example 6

Two aluminum plates were spray coated with a 5 % sodium silicate solution and excess solution was doctored off One plate "C" was baked for 15 seconds at 150 degrees C, while the other plate "D" was not so baked Both plates were then rinsed and dried and subjected to a zincate test.

Plate C produced a zincate reading of 85 whereas plate D produced a zincate reading of 57 indicating that plate C demonstrated superior protecting and bonding characteristics over the control plate D.

Claims (8)

WHAT WE CLAIM IS:

1 A method of producing a lithographic printing plate which comprises applying an aqueous coating solution of an alkali metal silicate, silicic acid, alkali metal zirconium fluoride, hydrofluozirconic acid, Group IVB metal fluoride, or polyacrylic acid, to at least one surface of an aluminium sheet, removing therefrom any excess solution thereafter subjecting the coating aluminium sheet to a hot air baking treatment at an elevated temperature and for a time sufficient to complete the reaction of the coating solution which has not reacted with the surface of the aluminium sheet and applying a light sensitive lithographic composition to the coated aluminium sheet.

2 A method as claimed in claim 1 where the solute concentration in the aqueous solution is from 0 5 % to 20 % by volume of total solution.

3 A method as claimed in claim 1 or 2 in which the coated aluminium sheet is subjected to an elevated temperature in the range of from 50 degrees C to 300 degrees C, for a period of from 5 to 120 seconds.

4 A method as claimed in any of claims 1 to 3 in which prior to coating the surface of the aluminium material has been anodized.

A method as claimed in any of claims 1 to 4 wherein the light sensitive lithographic composition is a diazo compound.

6 A lithographic printing plate produced by the'-method claimed in any of claims 1 to 5.

7 A method as claimed in claim 1 substantially as hereinbefore described in any one of the Examples.

8 A lithographic printing plate as claimed in claim 5 as hereinbefore described in any one of the Examples.

W.P THOMPSON, Coopers Building, Church Street, Liverpool, Li 3 AB.

Chartered Patent Agents.

Feinted for Her Majesty's Stationery Off ire, by Croydon Printing company Limited, Croydon, Surrey, 1981.

Published by The Patent Office, 25 Southampton Buildings.

London, WC 2 A l AY, from which copies may be obtained.